Equalizing Oil and Water Separation in Sample Tube

My son is working on a science project, and I am stuck in trying to figure out how to help him. He's doing conductivity testing in various liquids, and so what we started with was a small glass half full of water and half full of oil.

The glass will be replaced with a larger tank with valves etc to do sampling but the problem is this:

We would like to have the tank say the size of a beer keg, with a sample tube that is the size of a large Slurpee straw. How do we get the level of oil/water in the sample tube to match the level in the keg? When we tested this it was always the case that the water would rush in the bottom of the sample tube and fill it up, without the oil making it in.

I apologize if this sounds totally newbian, but I have looked all over for the answer to this but I am struggling to find one!